The present invention relates to a method for evaluating optical modulation characteristics of a liquid crystal modulation element, a liquid crystal display device produced by applying the foregoing method, a device for evaluating optical modulation characteristics of the liquid crystal modulation element by the foregoing evaluating method, and a computer-readable storage medium storing a program for evaluating optical modulating characteristics of a liquid crystal modulation element by the foregoing evaluating method.
A liquid crystal display (LCD) device is widely used as display device of an electronic apparatus, and the Jones matrix method is known as typical optical characteristic computing means. In this method, by expressing transmission of an electric field by light with a matrix with complex number elements in two rows and two columns (Jones matrix) and an input electric field of light with a complex vector with two rows and one column (Jones vector), and by operating the Jones matrix to the Jones vector, an electric field of the output light can be expressed and calculated in the same form of the Jones vector. This method is widely applied to optical designing of LCD elements since calculations conducted in the method are easy.
However, the foregoing designing method has two drawbacks shown below. First of all, (1) it is difficult to quickly understand a change in a polarization state with a phase difference, and (2) it is difficult to express a change in a polarization state of light in the case where a medium showing characteristics that may cause depolarization due to the scattering and the like is placed in a path of light.
The foregoing drawbacks can be solved by the Mueller matrix method that is another method of expression of polarization utilizing matrices. In this method, a polarization state is expressed with a Stokes""s vector with real number elements in four rows and one column, and a change in a polarization state is expressed by a matrix with real number elements in four rows and four column (Mueller matrix). In the case where a polarized light is in a pure state (monochromatic light that does not require statistical averaging), particularly, such a polarization state can be expressed as a point on a spherical surface (Poincarxc3xa9 Sphere) with its radius being proportional to an intensity of the light. A change in the polarization state due to transmission with a normal phase difference is expressed by rotational transform on the surface of the Poincarxc3xa9 sphere. By so doing, the foregoing drawback (1) can be solved, with such quickly-understood expression. Further, this method is characterized in that expression of light in a mixed state (a monochromatic light that requires statistical averaging), which means that transmission characteristics of a medium that scrambles polarization (for instance, a scatterer with a strong scattering effect) can be expressed with the Mueller matrices.
Regarding these natures about polarization, details are described in xe2x80x9cPolarized light: production and usexe2x80x9d, William A. Shurcliff, Harvard University Press, 1962.
Conventionally, the aforementioned Jones calculating method has been used for predicting optical characteristics of liquid crystal display elements. This process is generally as follows. First, an optical configuration of a liquid crystal element or the like is set, and optical characteristics are calculated by calculating means such as the Jones matrix method, etc., then, the designer adjusts the optical configuration, considering the result of calculation and other design factors. Further, expressing design factors predicted by the designer beforehand as design parameters and adopting values of design parameters that optimize final characteristics is generally conducted. A reflection-type single-polarizing-plate LCD device in which a single polarizing plate is provided on the observer""s side and a reflection plate is provided on the liquid crystal layer side, in particular, is disclosed in the Japanese Publication for Laid-Open Patent Application No. 236523/1990 (Tokukaihei 2-236523 [Date of Publication: Sep. 19, 1990]) (Japanese Patent No. 2616014) and the Japanese Publication for Laid-Open Patent Application No. 167708/1994 (Tokukaihei 6-167708 [Date of Publication: Jun. 14, 1994]).
According to conventional optical designs of liquid crystal modulation elements to which the foregoing designing method was applied, designing was conducted through the following flow. First, design parameters including characteristics and configurations of respective optical factors such as optical elements involved in display (polarizing elements such as a polarizing plate, a phase difference plate, a liquid crystal layer), control factors (voltages, etc.) for a liquid crystal layer, etc. are set to specific values intended by the designer. Then, transmittance (or reflectance) of light passing through the entirety of the optical element is calculated by means of Jones matrices or the like, and the result is judged by the designer. In the case where it is judged as inadequate, values of the design parameters are changed.
By the foregoing conventional designing method, however, even in the case where optimal solutions are obtained by correct calculations based on the optical principles in design parameter ranges assumed by the designer, a possibility that further better solution might be obtained in design parameter ranges outside the foregoing range cannot be denied. It follows that it is impossible to surely find the truly optimal setting.
Actually, as to the reflection-type signal-polarizing-plate liquid crystal display device disclosed, a range of the types of optical elements and a range of configurations thereof that the designer supposes are narrow.
More specifically, though calculations of polarization states are appropriately carried out in the foregoing Tokukaihei 2-236523, a guideline for designing a liquid crystal layer in cases including the case where a phase difference plate is used is not taught. Further, obtained in the publication is only an optimal solution in the case of an extremely restricted arrangement in which alignment (director) when a transmission axis (absorption axis) of a polarizing plate is parallel or orthogonal with respect to an alignment orientation of liquid crystal in an area of contact with a polarizing-plate-side substrate. Further, an optimal setting found in result is, in the case of liquid crystal alignment with twist, a setting in which the twist is 63xc2x0 and a product xcex94nd of a thickness of a liquid crystal layer (d) and a refraction index difference (xcex94n) is 193 nm. Further, a case where circularly polarized light enters the liquid crystal layer is taken as an example in the disclosure of the foregoing publication, and the foregoing setting is regarded as optimal in this case as well.
Incidentally, in the disclosure of the foregoing publication, a liquid crystal alignment is limited to the following specific case: among the two types of liquid crystal alignment applied for bright display and for dark display, respectively, one is horizontal alignment with uniform twist, while the other is completely vertical alignment. This means that a combination of voltages applied to the liquid crystal layer is limited to the ideal of 0V and an infinite voltage, and this is far different from voltages actually applied to a liquid crystal modulation element.
Further, in the foregoing Tokukaihei 6-167708, the type and position of provision of the phase difference plate is restricted, and cases where a different type of a phase difference plate is used or cases where the phase difference plate is disposed at a different position are not considered. Moreover, though the device is appropriately designed by considering liquid crystal alignment under actual voltage application, the liquid crystal alignment is limited to alignment without twist, and other general alignment of liquid crystal is not considered.
To execute optimization by using more numbers and ranges of design parameters by the foregoing conventional designing method, an increase in the number of design parameters leads to an increase in calculating operations since calculation has to be carried out with respect to each parameter by the foregoing method: with respect to the number N of design parameters including types and positions of optical elements such as a phase difference plate and alignment of a liquid crystal layer, a quantity of calculation increases in proportion to about N-th power. Consequently, the number of optical elements actually designable is limited.
In other words, in an LCD device such as a single-polarizing-plate reflection-type LCD device, inevitable characteristics of optical elements such as a polarizing element, a phase difference plate, and a liquid crystal layer are not expressed by a method with generality. Besides, a liquid crystal layer is not quantitatively designed by such a general method. Moreover, a method for quantitatively judging adequacy with respect to a result of such designing has not yet been taught.
An object of the present invention is to provide a method for evaluating optical modulation characteristics of a liquid crystal modulation element, which method is arranged as follows: an index indicative of a brightness modulation quantity as an optical modulation characteristic of a liquid crystal modulation element, namely, adequacy of an optical modulation characteristic, is expressed by a method with generality without being affected by the number of design parameters of optical elements to be determined at a final stage, thereby giving clear strategy to obtain optimal parameters of the optical elements. This gives a designer a relatively easy way to find conditions necessary to optimize the optical modulation characteristics.
Another object is to provide an LCD device produced according to the foregoing evaluating method. Still another object is to provide an evaluating device, and a computer-readable storage medium storing a program for executing the evaluating method.
To achieve the foregoing object, a method for evaluating optical modulation characteristics of a liquid crystal modulation element in accordance with the present invention is a method for evaluating optical modulation characteristics of a liquid crystal modulation element that includes one or a plurality of optical modulation elements for modulating a polarization state of light, one of the optical modulation elements being an optically anisotropic object having a liquid crystal layer, and the method is characterized by comprising the steps of:
given:
that optical modulation effects of the optical modulation element are expressed by Mueller matrices, respectively;
that the Mueller matrices are multiplied from left in a light transmission order so as to obtain a Mueller matrix given as MLCD;
that, concerning a control factor V=V1, V2 of the optical modulation element, one of the same is for dark display, and the other one is for bright display; and
that Mueller matrices regarding light since immediately before incidence to the optically anisotropic object until immediately after outgoing therefrom when V=V1 and when V=V2 are given as M(V1) and M(V2), respectively,
(a) deriving a Mueller matrix Mxcex1 expressed as:
Mxcex1=M(V2)M(V1)xe2x88x921
xe2x80x83and,
(b) evaluating optical modulation characteristics of the liquid crystal modulation element by:
expressing a QOM that is a predetermined quantity of the optically anisotropic object with elements of the Mxcex1;
arranging the QOM so as to be proportional to a difference between a value in the bright display and a value in the dark display of an (0,0)element of the MLCD, the (0,0)element indicating a brightness of the liquid crystal modulation element; and
using the QOM as an index for a brightness modulation quantity equivalent to a difference between brightness of the bright display and that of the dark display.
According to the foregoing arrangement, the QOM that is proportional to the (0,0)element of the foregoing MLCD that indicates a brightness difference in the case where the optically anisotropic object is combined with the polarizing element is used an index of a brightness modulation quantity, expressed with use of elements of the forgoing Mxcex1, whereby optical modulation characteristics of the liquid crystal modulation element are evaluated. In other words, a brightness modulation quantity as a normalized value after excluding brightness of dark display from brightness of bright display is expressed with use of a brightness difference in the case where the optically anisotropic object is used in combination with the polarizing element, so that the brightness modulation quantity is obtained. For instance, assume that dark display is obtained when a control factor V satisfies V=V1, while bright display is obtained when the control factor V satisfies V=V2.
In the case of a single-polarizing-plate reflection-type liquid crystal modulation element, an evaluation function QOM determined by the following expression is used for evaluating a brightness modulation quantity with use of a normalized difference between reflectances of bright display and dark display:   QOM  ≡      1    -                  {                              (                                                            M                                      1                    ⁢                    c                                    -                                ⁢                                  (                                      V                    2                                    )                                            ⁢                                                (                                                            M                                              1                        ⁢                        c                                                              ⁢                                          (                                              V                        1                                            )                                                        )                                                  -                  1                                                      )                    33                }            2      
Then, adequacy of optical design of the liquid crystal modulation element is judged with reference to the value thus obtained. Incidentally, M1c(V1) and M1c(V2) both are Mueller matrices to affect light with a specific wavelength that passes through a liquid crystal layer in a normal direction with respect to the layer that has a liquid crystal alignment controlled by a control factor V, and a subscript of xe2x80x9c33xe2x80x9d is indicative of a matrix element of the Mueller matrix that determines transform relationship of a circularly polarized component, that is, an (3,3)element.
Therefore, the foregoing QOM is expressed with only optical characteristics inside the foregoing optically anisotropic object including the liquid crystal layer, and a state of light before incidence to the optically anisotropic object may be affected by any additional polarizing effect due to a phase difference plate or the like.
Therefore, it is possible to express an index indicative of a brightness modulation quantity as an optical modulation characteristic of a liquid crystal modulation element, namely, adequacy of an optical modulation characteristic, by a method with generality that is not affected by the number of design parameters of optical elements to be determined at a final stage, thereby allowing an optical characteristic value to be easily obtained. Consequently, conditions of design parameters necessary for optimizing the optical modulation characteristics and for executing satisfactory optical modulation can be easily found.
Thus, by using the foregoing index, necessary conditions for satisfactory optical modulation can be expressed and evaluated by a method with generality irrespective of the number of optical elements and the types thereof. Therefore, optimal values of design parameters of optical elements can be easily found based on the evaluation result, and consequently, operations for adjusting values of the design parameters for optimization can be easily carried out.
For instance, in the case where the optical modulation element includes at least one linearly polarized light selective transmission element and takes advantage of external field response of the liquid crystalline material, the method may be further arranged by further including the step of controlling the liquid crystal layer and the control factor, in the set alignment of the liquid crystal used, with at least one combination of used control factors, namely, the foregoing control factors V1 and V2, and at at least one wavelength in a wavelength range of light used, so as to cause the QOM to have a value not less than a predetermined value, for instance, 0.9.
Furthermore, an LCD device in accordance with the present invention is an LCD device as a liquid crystal modulation element that includes one or a plurality of optical modulation elements for modulating a polarization state of light, one of the optical modulation elements being an optically anisotropic object having a liquid crystal layer, and the LCD device is characterized by being produced by:
given:
that optical modulation effects of the optical modulation element are expressed by Mueller matrices, respectively;
that the Mueller matrices are multiplied from left in a light transmission order so as to obtain a Mueller matrix given as MLCD;
that, concerning a control factor V=V1, V2 of the optical modulation element, one of the same is for dark display, and the other one is for bright display; and
that Mueller matrices regarding light since immediately before incidence to the optically anisotropic object until immediately after outgoing therefrom when V=V1 and when V=V2 are given as M(V1) and M(V2), respectively,
deriving a Mueller matrix Mxcex1 expressed as:
Mxcex1=M(V2)M(V1)xe2x88x921
xe2x80x83and,
evaluating the optical modulation characteristics of the liquid crystal modulation element, by:
expressing a QOM that is a predetermined quantity of the optically anisotropic object with elements of the Mxcex1;
arranging the QOM so as to be proportional to a difference between a value in the bright display and a value in the dark display of an (0,0)element of the MLCD, the (0,0)element indicating a brightness of the liquid crystal modulation element; and
using the QOM as an index for a brightness modulation quantity equivalent to a difference between brightness of the bright display and that of the dark display.
According to the foregoing arrangement, by using the foregoing evaluation function QOM as the index, necessary conditions for satisfactory optical modulation can be expressed and evaluated by a method with generality irrespective of the number of optical elements and the types thereof. Therefore, optimal values of design parameters of optical elements can be easily found based on the evaluation result, and operations for adjusting values of the design parameters for optimization can be easily carried out. Consequently, an LCD device with optimal optical modulation characteristics can be produced easily.
Furthermore, an evaluation device in accordance with the present invention for evaluating optical modulation characteristics of a liquid crystal modulation element includes one or a plurality of optical modulation elements for modulating a polarization state of light, one of the optical modulation elements being an optically anisotropic object having a liquid crystal layer, and the evaluation device is characterized by including:
given:
that optical modulation effects of the optical modulation element are expressed by Mueller matrices, respectively;
that the Mueller matrices are multiplied from left in a light transmission order so as to obtain a Mueller matrix given as MLCD;
that, concerning a control factor V=V1, V2 of the optical modulation element, one of the same is for dark display, and the other one is for bright display; and
that Mueller matrices regarding light since immediately before incidence to the optically anisotropic object until immediately after outgoing therefrom when V=V1 and when V=V2 are given as M(V1) and M(V2), respectively,
a matrix calculation section for deriving a Mueller matrix Mxcex1 expressed as:
Mxcex1=M(V2)M(V1)xe2x88x921
xe2x80x83and,
an evaluation section for evaluating the optical modulation characteristics of the liquid crystal modulation element, by:
expressing a QOM that is a predetermined quantity of the optically anisotropic object with elements of the Mxcex1;
arranging the QOM so as to be proportional to a difference between a value in the bright display and a value in the dark display of an (0,0)element of the MLCD, the (0,0)element indicating a brightness of the liquid crystal modulation element; and
using the QOM as an index for a brightness modulation quantity equivalent to a difference between brightness of the bright display and that of the dark display.
Furthermore, a computer-readable storage medium in accordance with the present invention is a computer-readable storage medium that stores a program for evaluating optical modulating characteristics of a liquid crystal modulation element, the liquid crystal modulation element including one or a plurality of optical modulation elements for modulating a polarization state of light, one of the optical modulation elements being a optically anisotropic object having a liquid crystal layer, is characterized by storing a program that is for evaluating optical modulation characteristics of the liquid crystal modulation element by:
given: p2 that optical modulation effects of the optical modulation element are expressed by Mueller matrices, respectively;
that the Mueller matrices are multiplied from left in a light transmission order so as to obtain a Mueller matrix given as MLCD;
that, concerning a control factor V=V1, V2 of the optical modulation element, one of the same is for dark display, and the other one is for bright display; and
that matrices regarding light since immediately before incidence to the optically anisotropic object until immediately after outgoing therefrom when V=V1 and when V=V2 are given as M(V1) and M(V2), respectively,
deriving a Mueller matrix Mxcex1 expressed as:
Mxcex1=M(V2)M(V1)xe2x88x921
xe2x80x83and,
evaluating the optical modulation characteristics of the liquid crystal modulation element, by:
expressing a QOM that is a predetermined quantity of the optically anisotropic object with elements of the Mxcex1;
arranging the QOM so as to be proportional to a difference between a value in the bright display and a value in the dark display of an (0,0)element of the MLCD, the (0,0)element indicating a brightness of the liquid crystal modulation element; and
using the QOM as an index for a brightness modulation quantity equivalent to a difference between brightness of the bright display and that of the dark display.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.